Competition for Nitrogen and Groundwater Nitrate Levels in Temperate Alley Cropping Systems

Competition for Nitrogen and Groundwater Nitrate Levels in Temperate Alley Cropping Systems

Summary

A three-year research project through the School of Forest Resources and Conservation, University of Florida, is studying how trees and crops compete for nitrogen within a temperate agroforestry system, and how groundwater nitrate levels are affected. Alley cropping systems with loblolly pine, longleaf pine and pecan, were interplanted with cotton, peanut or forages. Nutrient competition is being quantified via 15N enriched fertilizer movement and by variations in crop yields. Monthly changes in soil and groundwater nutrient levels are also being documented. Early findings indicate significant interspecific competition for nitrogen and water, with reduced groundwater nitrate levels when trees are nearby.

Objectives/Performance Targets

1. Quantify competition for nitrogen between trees and crops in temperate alley cropping systems using 15N labelled fertilizer

2. Examine ground water nitrate levels in alley cropping systems with and without tree-crop interactions

Accomplishments/Milestones

The agroforestry project is well underway. Five alley cropping systems have been established, and one-to-two growing season’s worth of yield and nutrient status data have been collected. A mature pecan-cotton alley cropping system was established in Spring 2001, as a principal research focus. The system is a RCBD consisting of 10 plots, of which 5 are “trenched” and 5 are “no trench” (i.e., control). Trenched plots have a 3 ft deep plastic liner installed at a distance of 5 ft from the center of tree, for the length of the plot, to prevent tree roots from interacting with the adjacent cotton In addition, tensiometers were installed in this system in selected plots (2 control and 2 trenched) to assess water potential differences at points just above and below the lysimeters. Regular readings are also being taken to determine volumetric water capacity of the soil. Nitrogen mineralization is also being quantified by the use of in-situ soil mineralization bags. To quantify nutrient competition, 15N fertilizer (ammonium sulfate, 5% enrichment) was applied to selected microplots at the same time, rate and formulation as the regular fertilizer being applied to the field. Six plants (above-ground portions only) from the center of each microplot were harvested at physiological maturity (early October), and were separated into stem, leaf, square, and boll components. Foliar samples from trees were also collected. Concentration of 15N is to be determined for both crop and tree plant tissues. Initial findings indicate that mature pecan trees are able to reduce the amount of nitrate in in-situ groundwater (Figure 1). Yields were also lower in untrenched plots, indicating competition for water from tree roots.

The agroforestry project has enjoyed important exposure in the scientific community, being the subject of a conference presentation by Samuel C. Allen, graduate researcher, who attended the 7th Biennial Conference on Agroforestry in North America, sponsored by the Association for Temperate Agroforestry (AFTA), in late August 2001. In addition, the research site was observed by local farmers and landowners who attended a Field Day in late 2001 at the Jay, FL, experiment station, where the research is conducted.

The agroforestry project (as carried out by Mr. Allen) is in its last year of data gathering and analysis. Soil and groundwater samples will continue to be collected through early 2002, and plant tissue analyses for 15N and other constituents will be analyzed. Remaining activities will involve analysis and write-up of research findings, with a target completion date of December 2002.

Impacts and Contributions/Outcomes

Outcome from the agroforestry project will be used to provide a profile of the interaction effects of trees and crops on the status of nitrogen and groundwater nitrate levels in temperate alley cropping systems. Findings from crop yields, soil moisture measurements, and water, soil and plant tissue analyses, will shed light on crop and tree root activity, and their effects on site environmental conditions. This knowledge will help to improve our basic understanding of temperate alley cropping systems, so that better systems can be created, with tighter nutrient cycling and reduced groundwater pollution.

Ultimately, it is hoped that this research will encourage farmers and landowners to adopt agroforestry practices. The researchers believe that such systems can help to diversify and strengthen the family farm, by providing alternate forms of income at various times of the year, while utilizing land that would otherwise remain unused. We also hope that landowners would be encouraged to plant longleaf pines on their property, given the possibility of incorporating this species with other more immediate cash crops. The prospect of reducing nitrate levels in groundwater is also an exciting possibility for these types of systems, which is a vision that we hope landowners are able to catch.

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